Wind turbines are becoming more prevalent as these are increasingly used as a source of electricity. A direct-drive wind turbine has several advantages over a conventional wind turbine that comprises a gearbox, since a direct-drive wind turbine requires fewer parts, is less complex and more reliable. For these reasons, demand for direct drive wind turbines is increasing.
A direct-drive generator has a relatively large diameter and many magnets of alternating polarity arranged along a circumference of a field magnet arrangement—usually an outside rotor—to allow for a sufficiently high pole-change frequency. The physical dimensions and weight of such a large generator pose problems during its assembly. Handling of the heavy, unwieldy and vulnerable components is complex and time-consuming, and is also hazardous, so that strict safety measures must be adhered to. This adds considerably to the overall time and cost required for the assembly of a direct-drive generator. In particular, the merging of a large rotor with a correspondingly large stator equipped with many windings can be problematic, since a very small separation—usually only a few millimeters wide—must be maintained between the rotor and stator during and after the merging. For an interior stator inserted into an outside rotor, care must be taken to avoid a collision between the windings on the outside surface of the stator and the interior surface of the rotor.
U.S. Pat. No. 4,157,613 and JP 61 128 747 A describe apparatus for assembling a medium-sized generator with inner rotor. These techniques make use of the existing rotor shaft to facilitate assembly, in that the ends of the rotor shaft are held by holding means that guides the rotor into the outer stator. However, large generators of the type described above do not have a central shaft, and the interior of such a generator often comprises a relatively large access cavity. Usually therefore, in prior art assembly methods for a generator with outer rotor, the stator, weighing several tonnes, is suspended or held by a crane or other lifting equipment and carefully introduced into the interior space of the rotor. Even though care is taken to maintain the required separation between the rotor and the stator, the risk of damage to the windings or to the interior rotor surface is very high, and any repairs are time-intensive as well as and cost-intensive.